Many processes for making water formed paper webs or dry formed nonwoven webs include the application of a bonding material to the web to give the web either stability or strength. In such cases, it is often necessary to set or cure the bonding material by raising the temperature of the bonded web. One example of such a process is described in U.S. Pat. No. 3,879,257, issued to Victor R. Gentile, et al, and assigned to the assignee of this invention.
One method for curing a bonded, air-permeable web is to transport the web through a curing dryer in which heated air is passed through the web. In such a dryer, the three primary parameters that affect the degree of cure of the bonding material are the dwell time of the web in the curing dryer, the temperature of the air passing through the web and the velocity of the heated air passing through the web. Typically, the dwell time of the web in the curing dryer is constant because the web travels at a constant speed through the dryer, the temperature is controlled to a predetermined temperature, and the air flow control valve is set to a predescribed position to effect curing of the web. In the prior art curing systems, these parameters were adjusted manually by an operator who tests the degree of cure of the finished web and who changes either the temperature setting or air flow setting based on his observation. The cure test is usually performed by the operator after a parent roll of the finished web has been completed, which in a wet formed papermaking process occurs about once every 10 or 15 minutes. One problem with a manually controlled curing system is that certain properties of the web can change over relatively short periods of time. For example, if the web is a wet formed paper web, either the basis weight or the bulk of the web can change over relatively short periods of time (measured in seconds), either because of changes in the papermaking process itself, or because either the chemical or fiber composition of the furnish in the wet end changes. Changes in the bulk, basis weight or fiber composition of the web usually results in a change in the permeability of the bonded web which in turn, for a given air flow, affects the velocity at which the heated air passes through the web while it is in the curing dryer. The primary problem associated with attempting to provide on-line control of the parameters that affect the degree of cure of the web is the fact that it is extremely difficult to directly measure the velocity of the heated air passing through the web in the dryer. The reason for this difficulty is that, due to leaks and losses, it is not possible to obtain an accurate measure of the actual volume of heated air passing through the web, and because of variations in the trim of the web it is not possible to accurately measure the area of the web through which the heated air passes.
Thus, it can be seen that it would be desirable to have a curing apparatus in which heated air is passed through the web for a predetermined time and a predetermined temperature, and in which a property of web is measured and used to control the flow of heated air through the web in the dryer so as to maintain a desired velocity of the heated air through the web.